The advent of molecularly targeted anticancer therapies has energized the cancer field and given new hope to cancer patients. Discovery of intracellular pathways that can lead to tumor growth suppression and the development of monoclonal antibodies that specifically target cancer pathways are being developed and tested in human clinical trials. The dawn of such targeted anticancer therapies could cause a certain lack of interest in the development of novel conventional cytotoxic small molecule drugs. However, molecularly targeted therapies have not shown improved curative properties when administered as monotherapy [1]. As a result, most molecularly targeted agents today are used in combination with cytotoxic compounds.
In contrast to the tumor-static activities of many targeted agents, cytotoxic agents have the ability to directly kill cancer cells through their inhibition of critical cell growth pathways and exert a broader range of activity as compared to targeted therapies that are inherently narrow. For these reasons, there is hope in combining molecular targeted anticancer therapies with conventional cytotoxic chemotherapy [2, 3]. Indeed, current standard of care treatments are based on sequential therapies by combining multiple drugs with the intent of overcoming drug-resistance, which is typical of many tumors and a major issue in fighting cancer. In the age of targeted therapies and therapeutic monoclonal antibodies, it is noteworthy that extensive resources and scores of clinical trials are still being devoted to the identification and evaluation of small molecules chemotherapy agents, thus reiterating the need for improved chemotherapy drugs. This is partly due to an extremely large untapped reservoir of potential therapeutic compounds which tackle several biological aspects of a highly proliferating living cell and also to the growing understanding of the life cycle of cancer cells and onset of drug-resistance. Overall, there is still a need to develop more effective cytotoxic drugs. These new chemotherapy drugs need to show an increased potency, low systemic toxicity, better bioavailability and that can be used in combination with current approved drugs.
Among cancers, the high incidence and mortality of colorectal cancer is a major health issues worldwide [4, 5]. While the ability to treat patients with colorectal cancer has improved in the past 15 years, thanks to optimized surgery techniques, better radiotherapy and novel chemotherapy drugs, treatments are still not ideal. Major toxicities (grade 3 or 4) are still a problem with standard-of-care chemotherapy drugs. SFU, irinotecan, leucovorin, oxaliplatin, either alone or in the FOLFOX or FOLFIRI regimens, result in severe side effects that can also developed into neuronal toxicity [6, 7]. Targeted therapies displayed overall less side effects events when administered alone, however, combinations with chemotherapy drugs is typically more effective in colorectal patients, especially when RAS/RAF mutations are present. For the latter, the identification of KRAS and BRAF mutations in colorectal cancer patients has become a crucial diagnostic factor for designing effective therapy regimens that could treat also metastatic colorectal cancers [8, 9].
Additionally, melanoma cancer rates have risen steadily for the last 30 years. Nonetheless, the median survival is <1 year and the 5 year survival rate is 10%. [30, 31]. Two targeted therapies were approved in 2011, ipilimumab (a monoclonal antibody) and vemurafenib (a BRAF kinase inhibitor), that exhibit robust anti-tumor activity, but patients relapse with lethal drug resistant disease within 5-7 months. [32-35] The biological response modifier adesleukin (recombinant interleukin-2) has a 50-60% response rate, but can produce life-threatening toxicities. The response rate for temozolomide, a DNA modifying agent, is <20%.
Breast cancer is a major cause of cancer-related death in women preceded only by lung cancer. [43] The expression levels of estrogen receptors (ER) and progesterone receptors (PR) as well as the amplification status of the HER-2/Neu gene help direct diagnosis and treatment of breast cancer. For tumors that express one or more of these biomarkers, targeted therapies have significantly improved patient outcomes. However, breast cancers lacking the aforementioned biomarkers, termed triple negative breast cancer (TNBC), present severe challenges for patient survival since biomarker-targeted therapies are ineffective.
As discussed above, current therapies are not always effective, and as such, there is a need for improved drugs with higher activity for treating cancers with minimum side effects that can be used in alone, combinational and/or sequential treatments.